This invention relates generally to controls for air treatment units and particularly to controls for pressure equilibrium modulated air intake systems which combine treated air with a modulated composite air supply of untreated outside air and air drawn from within the treated space. The invention is particularly adapted for use with direct fire air heaters.
U.S. Pat. No. 4,429,679, issued to James V. Dirkes for a MODULAIR AIR HEATER and assigned to assignee of the present application discloses a pressurized space heating apparatus wherein a fixed portion of outside air is supplied to a direct fire burner to be heated and another portion is mixed in complimentary proportions with bypassed air that is drawn from the heated space. The two portions are combined downstream of the burner to maintain space air at a substantially uniform temperature. The pressure within the space is measured with respect to outdoor air pressure. The relative pressure is used to continuously alter the complimentary proportions of unheated outside air to bypassed air in order to modulate the fixed volume of heated air and thereby maintain a fixed, slightly positive, relative pressure in the space to be heated. The air heater disclosed in Dirkes finds application primarily in industrial units such as warehouses, factories and the like. Because the control responds almost instantaneously to a condition affecting the pressure balance within the heated space, by adjusting dampers to restore equilibrium, situations can arise which result in excessive energy loss. For example, when a large opening is created in the envelope of the space, for example, when a freight door is left open, the control will respond to the decrease in positive pressure within the space by modulating the dampers to bring in more outside air in order to increase the positive pressure within the space. Because the amount of air exiting the space must balance the infiltration of outdoor air, the result is a significant increase in the exchange of heated air from the space with the outdoors. This aggravates the loss of energy otherwise resulting from the open door.